Precision frequency counters typically include two counters, a time counter to accumulate events from a stable clock, and an event counter to accumulate events from an input signal. Dividing the number of events in the event counter by the result in the time counter provides the average frequency of the input signal.
The two counters are started and stopped by a signal called a gate. If the gate is synchronized with the clock, the gate time is controlled exactly and the number of input signal events is counted with some uncertainty (plus or minus one event). If the gate is synchronized with the input signal, the number of input signal events is counted exactly and the elapsed time is counted with some uncertainty (plus or minus one clock period). This latter method is called reciprocal counting.
Because the plus or minus one event count uncertainty gives relatively poor frequency resolution, virtually all modern precision frequency counters use the reciprocal counting method. Increased accuracy can be gained by averaging over more than one input signal event.
However, division is an inherently slow operation. In the digital circuits used in most modern instruments, division requires many processing cycles or a very large amount of circuitry because the computation of each output bit uses a carry through many bits of the intermediate results of the previous output bit.
One approach to avoiding division is to measure the period between successive pulses and generate the inverse of the period to produce the frequency. This approach is disclosed in U.S. Pat. No. 4,707,653 (Wagner). Wagner uses a PROM which includes a lookup table to provide the inverse of the period. However, Wagner does not address making measurements over multiple cycles, which is necessary to produce high precision frequency measurements. Also, because Wagner's device measures on every input event, it is limited to relatively low frequency applications. Finally, the simple look up table limits the accuracy provided.
The limits imposed by division for measurements on multiple cycles made it difficult or impossible to make successive frequency measurements as rapidly as desired. Making more rapid frequency measurements would allow triggering a frequency versus time measurement on a frequency transition, making and displaying nearly real time frequency measurements of a frequency modulated or frequency agile signal, or producing histograms of frequency distribution of such signals. Another application is making a precision, wideband, programmable frequency to voltage converter.
Another application for making rapid frequency measurements is in an instrument for providing continuous time interval measurements on a signal. Continuous time interval measurements make it simpler to study dynamic frequency behavior of a signal: frequency drift over time of an oscillator, the frequency hopping performance of an agile transmitter, chirp linearity and phase switching in radar systems. Continuous time interval measurements on a signal provide a way to analyze characteristics of the signal in the modulation domain, i.e., the behavior of the frequency or phase of the signal versus time. An example of an instrument that generates this type of time stamp and continuous time interval data is described in "Frequency and Time Interval Analyzer Measurement Hardware", Paul S. Stephenson, Hewlett-Packard Journal, Vol. 40, No. 1, February, 1989.